Lehigh Septic, Inc.
PO Box 487,
Alva FL 33920

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What’s a “BIOMAT” — What is a septic drainfield “Biomat” Layer?

The biomat is a bacteria layer which forms in soil below and around drainfield trenches (areas) where septic effluent or wastewater is discharged. This layer is critical in the processing of fine biological solids and pathogens which are in the effluent. Without it the septic system would not be adequately treating the effluent. Inadequately-treated effluent released into the ground risks contamination of nearby ponds, wells, streams, etc. A similar layer also forms around drywells used to accept gray water from buildings.

Septic effluent, or onsite wastewater, is discharged into a soil absorption system, (or drainfield, drainbed, seepage pit, or cesspool) from the septic tank which should, if it’s working properly, have retained all large solids. The job of the soil absorption system, or “SAS”, is to further treat the effluent to reduce the level of biological solids and pathogens to a level acceptable for further movement of the liquid into remaining soils. Inadequate treatment of effluent would mean that sewage and pathogens would be discharged into and contaminate nearby ground water.

How the BIOMAT Forms — How does the Biomat Form in the Absorption System?

As the effluent is discharged into the SAS, bacterial growth develops beneath the distribution lines where they meet the gravel or soil. This layer is known as the clogging mat, clogging zone, bio crust and bio format.

It’s also referred to by some as the “slime layer” and it’s easily visible as a usually gray, slimy layer in the soil displayed if one excavates a cross-section of an absorption system trench.

This biomat is a black, jelly-like layer that forms along the bottom and the sidewalls of the drainbed. This clogging zone (eventually) reduces infiltration of wastewater into the (surrounding) soils.

BIOMAT CONSTITUENTS — What is the Biomat Made from?

The biomat is composed of anaerobic microorganisms (and their by-products) that anchor themselves to soil and rock particles.

“Anaerobic” refers to microorganisms which do not require high levels of oxygen, as opposed to “aerobic” organisms which do. Septic waste and wastewater treatment involves both aerobic and anaerobic organisms. “Aerobic” bacteria require oxygen.